Efforts have been made since the late 1960s to replace the high levels of phosphate builders in household detergent products with non-phosphate ingredients which fulfill builder functions without causing environmental damage.
Builders in automatic dishwashing products function to (1) provide alkalinity, (2) sequester hardness ions and (3) disperse soils so as to prevent redeposition on clean ware surfaces. Sodium carbonate has been used as a phosphate builder alternative affording a cost effective source of alkalinity and functioning to lower the free calcium ion concentration in the wash solution. However, sodium carbonate has the tendency to deposit calcite crystals or other forms of calcium carbonate in hard water and thus to cover both tableware and dishwasher interiors with a white crust. This problem persists even when sodium carbonate is used in combination with sodium citrate.
When carbonate products are used in hard water, encrustation is believed to result via the formation of invisible minute calcite crystal nuclei which then grow to visible size. In a super-saturated solution of calcium carbonate, nucleation occurs during all washes but after a few washes all surfaces in the dishwasher are covered with growing crystals and additional calcium carbonate crystallizes on those crystals already present. It is believed that sequesterants such as sodium citrate prevent the formation of amorphous calcium carbonate.
As early as 1936, U.S. Pat. No. 2,264,103 was issued for a process of softening hard water using certain organic acid salts including citric acid U.S. Pat. No. 4,102,799 disclosed a dishwasher detergent composition consisting essentially of a citrate builder salt in combination with at least one additional builder salt such as silicate, carbonate, etc. GB 1,325,645 also disclosed a dishwasher composition comprising an alkali metal salt of citric acid, alkali metal carbonate and other components.
As noted above although sodium citrate prevents the formation of amorphous calcium carbonate, once calcite crystals are present, the citrate rapidly loses most of its calcium ions to the calcite.
Therefore, anti-nucleation agents also termed anti-scalants, or scale inhibitors have been used to inhibit the development of microscopic nuclei which grow to visible size and then the anti-nucleation agents redisperse to act on other nuclei. The inhibition of calcite crystal growth can prevent encrustation. Polyphosphates, phosphonates, polysulfonates and polycarboxylate polymers are also known in the art to reduce calcium carbonate deposition from detergent products which are built with sodium carbonate.
Ideally, therefore, a zero-P or low phosphorus powder detergent contains a sequestrant, such as citrate; an inexpensive source of alkalinity such as sodium carbonate and an anti-scalant or scale inhibitor such as polycarboxylate, phosphonate or polysulfonate.
Anti-scalants which are presently available are in aqueous form or powdered forms having a particle size which passes through a 50 mesh U.S. Screen. Particle sizes which pass through a 14 mesh U.S. Screen and are no larger than .alpha. 50 mesh U.S. Screen are however desirable for the invention. Since commercially available anti-scalants do not fit these criteria novel processing methods were required to overcome these problems.
Unfortunately, it has been found that many suitable anti-scalants which are available are provided in their acid forms, as partially neutralized acids, or otherwise contain a free acid. The presence of acidic species in anti-scalants poses a problem in the manufacture of dishwasher detergents. Specifically, if such acidic species are not neutralized, but sprayed directly on the detergent ingredients which include silicate, it is known that the acidic constituent has a destabilizing effect on the silicate component to liberate insoluble silica. This effect was believed to be specific for solid silicates as discussed in U.S. Pat. No. 4,379,069 (Rapisarda et al.).
It has now been found a similar effect can occur with aqueous silicates. An additional problem associated with aqueous anti-scalants, whether acidic or neutralized, is the high level of water (about 40 to 60%) these anti-scalants contain. In detergent manufacturing, non-phosphate builders generally do not have the absorptive capacity of the phosphate builder nor do they generally form stable hydrates in manufacture. For example, sodium citrate is generally used in either its dehydrate form or anhydrous form. When relatively high levels of anti-scalant are required for a product, and the anti-scalants are in aqueous form, prolonged drying times are required to remove excess water resulting in high cost for energy and the reducing in manufacturing efficiency.